1. Field of the Invention
The present invention relates to a diamond film depositing apparatus and a method thereof, and more particularly, to a diamond film depositing apparatus and a method thereof which are capable of depositing a diamond thick film having a thickness of more than hundreds of xcexcm on a large substrate having a diameter larger than 4 inches.
2. Description of the Conventional Art
A DC PACVD (Direct Current Plasma Assisted Chemical Vapor Deposition) method, one of the generally known diamond film deposition methods, is notable for its advantages in that the apparatus for DC PACVD is simple and its depositing speed is faster than hot filament or microwave CVD techniques.
It is constructed that a pair of platy electrodes are disposed, facing parallel to each other within a vacuum in a reaction container and a DC voltage is applied between the two electrodes to generate a plasma, and this plasma is used to ionize a reactive gas introduced into the reaction container, so that a diamond film is deposited on a substrate placed on an anode. In this respect, a disk-type electrode is largely used, and the reactive gas is a mixture of hydrogen and hydrocarbon. However, this technique suffers problems in that a long-time deposition is difficult due to instability or a loss of plasma caused by arc generation, and in that it is hardly possible to enlarge the area of a uniform deposition larger than 1 inch by using a single cathode. These problems prevent synthesizing a diamond thick film having a diameter larger than 4 inches and a thickness of more than hundreds of xcexcm using a single cathode. That is, in the conventional art, it has been difficult to form a large plasma by using a single cathode for a large substrate having a diameter of larger than 4 inches, and even if such a plasma was successfully formed, it was difficult to keep it stable for a long time.
In the DC PACVD method, the temperature of the cathode and that of the substrate work as critical deposition variables. Varying the temperature of the cathode and the substrate over a wide range is very important during depositing. To be described in detail later in the description of the present invention, the cathode and substrate should be maintained not to go beyond a predetermined temperature range. The cathode is heated by the collision of ions from the plasma and the substrate by the collision of electrons. In a given structure for a cathode and a holder, the temperatures of the cathode and substrate are varied in accordance with the process conditions. In the conventional art, a general method for varying the temperatures of the cathode and substrate is seen in a manner that the cathode and the substrate are mounted on a water-cooled holder and a spacer is inserted between the holder and the cathode or the substrate, to thereby control the heat transfer. In this respect, if the temperature of the cathode and substrate could be varied by external operation outside a chamber during the process, it would be very convenient because there would be no need to discontinue the process to change the spacer. However as to the DC PACVD method, no technique has been reported to vary the temperatures of a cathode and a substrate during the process of depositing.
Conventionally deposition of a diamond thick film of hundreds of xcexcm in thickness regarding the single-cathode DC PACVD is only obtained when the deposition area is quite small, about 1 square cm. On the other hand, where deposition of a wide area as large as 4 inches in diameter was achieved, it was not the desirous diamond thick film having the thickness of hundreds of xcexcm, nor the continuous film but merely the formation of diamond particles, for which the plasma is rotated by applying a magnetic field. In other disclosed cases of attaining both the enlargement of area and the thick film deposition, employed a hot filament method was and used a plurality of filaments as a cathode. This method does not use a pair of facing plate-shape electrodes, and thus has an intrinsic problem of difficulty in filament maintenance, the basic defects in the hot filament technique.
Most of the conventional art techniques employ a ballast resistance connected in series to the power supply which serves to stabilize the plasma. The ballast resistance, having a similar resistance value to a load value of the plasma, is disadvantageous in that it unnecessarily wastes power because of its series connection to the circuit. That is, a large of percentage of the supply power is wasted in the ballast resistance, deteriorating the power efficiency. Nonetheless, the reason why the ballast resistance is in frequent use is that the plasma of the DC PACVD is hardly maintained stably for a long time using only the power supply.
The most conventional DC PACVD methods employ a flat-plate-type substrate. Meanwhile, some fields such as for radomes or diaphragms require that the diamond be deposited on a spherical surface. In addition, in some cases, deposition is needed on a cylindrical curved surface such as the internal and external surfaces of a tube, or an external surface of a bar. The conventional art of DC PACVD using a pair of electrodes does not disclose such a diamond deposition performed on a curved surface.
In the conventional DC PACVD methods, a thick plate of high melting point material such as molybdenum or tungsten is widely utilized as a substrate. If a silicon substrate in general use for the semiconductor fabrication is applicable thereto, it may be possible to be applied to the fields of SOD (Silicon On Diamond) and SAW (Surface Acoustic Wave) devices. Yet, there is not known a case where the diamond film is deposited on a silicon substrate having a diameter of 4 inches. The reason is believed to be that since the silicon substrate has a low thermal conductivity, being thin and light, a thermal contact between the silicon substrate and a substrate holder is not effectively made, resulting in that maintaining of an even temperature is impossible during synthesizing.
Therefore, the present invention is directed to provide a diamond depositing apparatus which is capable of forming a uniform and large plasma on a substrate having a diameter of over 100 mm and keeping it stable, without using a hot filament as a cathode, without applying an additional magnetic field, and without using a ballast resistance, to thereby deposit a diamond thick film having a thickness of over hundreds of xcexcm and having a diameter of larger than 4 inches on a flat-surfaced substrate, a curve-surfaced substrate and on a Si wafer, and to a method thereof.
To achieve these and other advantages, the diamond film depositing apparatus in accordance with the present invention, as embodied and broadly described herein comprises: a process chamber having a gas inlet that injects a reactive gas and a gas exhaust outlet that discharges an exhaust gas; a cathode disposed at an internal upper portion of the process chamber; an anode for absorbedly fixing a substrate, the anode being disposed at a lower portion of the cathode; a switched-mode power supply(SMPS) for forming a plasma between the cathode and the anode by applying a DC voltage to the cathode and the anode, the SMPS being connected to both the cathode and the anode; and a holder for fixing the cathode to the upper portion of the process chamber and controlling a temperature of the cathode, wherein the size of the cathode and the anode is larger than 100 mm in diameter.
There is also provided a diamond film depositing method comprising the steps of: applying a DC or DC pulse voltage to a cathode and the anode disposed inside a vacuum chamber; generating a plasma between the cathode and the anode by supplying a reactive gas therebetween; and depositing a diamond film on a substrate by constantly maintaining a gas pressure, while keeping the temperature of the cathode below 2000xc2x0 C.